Mechanisms of the aerobic oxidations catalyzed by N-hydroxyderivatives: Enthalpic, polar and solvent effects, “molecule-induced homolysis” and synthetic involvements

Aminoxyl (R2NO), amidoxyl (RCON(O)R) and imidoxyl ((RCO)2NO) radicals play a key role in the aerobic oxidation catalyzed by N-hydroxyderivatives. The rationalization of the mechanisms of a variety of oxidations is based on thermochemical, kinetic and spectroscopic investigations and on solvent effects and it has suggested new selective synthetic developments. In collaboration with CIBA Speciality Chemicals a new catalytic system resulting from the combination of a persistent macrocyclic aminoxyl radical and the couple Mn(NO3)2/Co(NO3)2 has been developed; it is particularly effective for the aerobic oxidation of alcohols to aldehydes and ketones under mild conditions (air at room temperature and atmospheric pressure); above all it presents the great advantage of an easy recovery and recycling providing the possibility of practical applications. The kinetic investigation of the substituent effect in the oxidation of benzyl alcohols has allowed identifying the rate-determining step of the oxidation. The amidoxyl radicals, generated “in situ” from N-hydroxyamide, revealed particularly effective catalysts for the aerobic peroxidation of polyunsaturated fatty acids and esters, which is involved in the origin of several important pathologies, such as tumor initiation and atherosclerosis. The kinetic investigation has contributed to explain the mechanism of the oxidation and to develop the most effective methodology for the synthesis of hydroperoxides. The importance of enthalpic, polar, captodative, solvent effects and “molecule-induced homolysis” has been emphasized in the oxidation, halogenation and acetoxylation of a variety of classes of organic compounds (hydrocarbons, alcohols, aldehydes, amines, amides, silanes) by O2 and N-hydroxyimide catalysis. The high selectivity often observed and the very mild experimental conditions, based on the mechanistic interpretation, provide industrial potentiality to the catalytic processes.

Graphical abstractAminoxyl (R2NO), amidoxyl (RCON(O)R) and imidoxyl ((RCO)2NO) radicals have resulted to be particularly effective as catalysts in the aerobic oxidations. Thermochemical and kinetic studies have allowed to develop innovative synthesis of derivatives of industrial and biological relevance through the selective oxidation of a wide range of hydrocarbons. Figure optionsDownload full-size imageDownload as PowerPoint slide